Make sure CFG_RELEASE_CHANNEL is always set.
Make sure CFG_RELEASE_CHANNEL is always set.

Previously we'd build libsyntax without it when documenting and that'd
cause us to recompile it when building normally for no reason.

aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,aidancully/rust